Standard sand blasting equipment consists of a pressure vessel or blast pot to hold particles of a blasting medium such as sand, connected to a source of compressed air by means of a hose and having a means of metering the blasting medium from the blast pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the size of the equipment. Commercially available sand blasting apparatus typically employ media flow rates of 20-30 pounds per minute. About 1.2 pounds of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 1.20.
When it is required to remove coatings such as paint or to clean surfaces such as aluminum, magnesium, plastic composites and the like, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate, can be used in conventional sand blasting equipment. The medium flow rates required for the less aggressive abrasives is substantially less than that used for sand blasting, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. This requires a much lower medium to air ratio, in the range of about 0.05 to 0.25.
However, difficulties are encountered in maintaining continuous flow at these low flow rates when conventional sand blasting equipment is employed. The fine particles of a medium such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, they tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but a substantially uniform flow of bicarbonate material to the nozzle has not been possible up till now. Sporadic flow of the blasting media leads to erratic performance, which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Thus it is desired to have a blasting apparatus that can deliver the blast media at a uniform rate that can be controlled in a predictable manner, at flow rates yielding a medium-to-air ratio of between about 0.05 and 0.25 by weight, using a configuration similar to conventional commercially available sand blasting equipment.